Nitroreductases

Ran J, A Stolz (2003) Oxygen-insensitive nitroreductases NfsA and NfsB of Escherichia coli function under anaerobic conditions as lawsone-dependent azo reductases. Appl Environ Microbiol 69 3448-3455. 

It has been shown that the nitroreductases from Escherichia coli and Vibrio harveyi are homologous to the chromate reductase from Pseudomonas ambigua (Kwak et al. 2003). 

Bryant C, L Hubbard, WD McFlroy (1991) Cloning, nucleotide sequence, and expression of nitroreductase gene from Enterobacter cloacae. J Biol Chem 266 4126-4130. 

Bryant C, M DeLuca (1991) Purification and characterization of an oxygen-insensitive N AD(P)H nitroreductase from Enterobacter cloacae. J Biol Chem 266 4119-4125. 

Koziarz JWP, J Veall, N Sandhu, P Kumar, B Hoecher, IB Lambert (1998) Oxygen-insensitive nitroreductases analysis of the roles of nfsA and nfsB in development of resistance to 5-nitrofuran derivatives in Escherichia coli. J Bacteriol 180 5529-5539. 

Kwak YH, DS Lee, HB Kim (2003) Vibrio harveyi nitroreductase is also a chromate reductase. Appl Environ Microbiol 69 4390-4395. 

Reduction of 5-nitrofnran derivatives associated with mutation in aryl nitroreductases (Whiteway et al. 1998). 

Somerville CC, SF Nishino, JC Spain (1995) Purification and characterization of nitrobenzene nitroreductase from Pseudomonas pseudoalcaligenes JS45. J Bacteriol 111 3837-3842. 

Rafii F, CE Cerniglia (1993) Comparison of the azoreductase and nitroreductase from Clostridium perfrin-gens. Appl Environ Microbiol 59 1731-1734. 

The third study has employed 4,6-dinitrobenzofuroxan and as metabolic systems the one-electron reductants NADPHxytochrome P450 reductase and ferredoxin NADP(+) reductase and the two-electron reductants DT-diaphorase and Enterobacter cloacae nitroreductase [239]. The compound is activated either by DT-diaphorase or nitroreductase. 

Nitroreductase Reduction of aromatic and heterocyclic p-nitrobenzoic adds,... 

Salmonella typhimurium. Although most nitro PAHs are direct-acting mutagens in Salmonella typhimurium, these compounds must be metabolized to bind covalently to DNA (71,92,112). S. typhimurium contains a family of nitroreductases which are capable of reducing nitro PAHs, and strains which are deficient in these enzymes generally show decreased sensitivity toward nitro PAH-induced mutations (27,92,113-114). These observations suggest that reduced metabolic intermediates may be the critical reactive electrophiles. 

Compared to the extensive data that have been obtained on the mutagenicity of nitro PAHs in S. typhimurium, relatively little is known about the metabolism of these compounds in this organism. Messier et al. (67) reported that incubation of 1-nitropyrene with S. typhimurium TA98 yielded 1-aminopyrene and 1-acetylaminopyrene as major and minor metabolites, respectively. The reduction of 1-nitropyrene was slow and was accompanied by a slow formation of DNA adducts. When incubations were conducted with the nitroreductase-deficient strain, TA100 F50, both the extent of 1-amino-pyrene formation and DNA binding decreased. Howard ej al. (71,115) also found reduction of 1-nitropyrene to 1-aminopyrene in strains TA98, TA1538 and ATCC 14028. 

Certain nitro PAHs appear to require metabolism of their N-hydroxy arylamine derivatives in order to induce mutations. For example, while 2-nitrofluorene showed decreased mutagenicity in the nitroreductase-deficient mutant, TA98NR, and in strain TA98/1,8-DNP, its presumed ultimate mutagenic derivative, N-hydroxy-2-aminofluorene was inactive in only strain TA98/1,8-DNP (117). Observations such as these led McCoy t al. (117) to... 

Other bacteria. Intestinal bacteria may play a critical role in the metabolic activation of certain nitroaromatic compounds in animals (119) and several reports have appeared on the metabolism of nitro PAHs by rat and human intestinal contents and microflora (120-123). Kinouchi et al. (120) found that 1-nitropyrene was reduced to 1-aminopyrene when incubated with human feces or anaerobic bacteria. More recently, Kinouchi and Ohnishi (121) isolated four nitroreductases from one of these anaerobic bacteria (Bacteroides fragilis). Each nitroreductase was capable of converting 1-nitropyrene into 1-aminopyrene, and one form catalyzed the formation of a reactive intermediate capable of binding DNA. Howard ej al. (116) confirmed the reduction of 1-nitropyrene to 1-aminopyrene by both mixed and purified cultures of intestinal bacteria. Two additional metabolites were also detected, one of which appeared to be 1-hydroxypyrene. Recently, similar experiments have demonstrated the rapid reduction of 6-nitro-BaP to 6-amino-BaP (123). 

Saito et al. (134) found that the cytosolic nitroreductase activity was due to DT-diaphorase, aldehyde oxidase, xanthine oxidase plus other unidentified nitroreductases. As anticipated, the microsomal reduction of 1-nitropyrene was inhibited by 0 and stimulated by FMN which was attributed to this cofactor acting as an electron shuttle between NADPH-cytochrome P-450 reductase and cytochrome P-450. Carbon monoxide and type II cytochrome P-450 inhibitors decreased the rate of nitroreduction which was consistent with the involvement of cytochrome P-450. Induction of cytochromes P-450 increased rates of 1-aminopyrene formation and nitroreduction was demonstrated in a reconstituted cytochrome P-450 system, with isozyme P-448-IId catalyzing the reduction most efficiently. 

Fig. 8.17. Mechanism of bioactivation of self-immolative carbamate prodrugs of cytotoxic amines. A bacterial nitroreductase coupled to a tumor-directed antibody reduces the prodrug to a hydroxylamine (Reaction a), which breaks down spontaneously to liberate the antitumor...

A. B. Mauger, P. J. Burke, H. H. Somani, F. Friedlos, R. J. Knox, Self-Immolative Prodrugs Candidates for Antibody-Directed Enzyme Prodrug Therapy in Conjunction with a Nitroreductase Enzyme , J. Med. Chem. 1994, 37, 3452-3458. 

A similar enzyme (nitroreductase) from the same bacterium was transferred into tobacco by Hannink et al. (2001). This enzyme utilises NAD(P)H to reduce TNT to aminodinitrotoluenes (ADNTs). Transgenic plants showed an enhanced tolerance and detoxification of TNT, although the metabolites could not be identified. 

The enzymatic system participating in degradation of TNT or other nitroaromatics in plants has not yet been sufficiently characterised. Based on recent results, TNT is not metabolised by highly specific nitroreductases, which would be purposefully synthesized by plants, but by constitutive enzymes with nitroreductase activity (Nepovim 2005b). This assumption is in agreement with results presented by Ekman (2005) showed an increase of the expression of a couple of enzymes in root mRNA due to the induction caused by explosives. 

Redox reaetions and hydrolysis are the predominant metabolie eonversions triggered by the intestinal microflora. The main reductive enzymes produeed by the intestinal mieroflora are nitroreductase, deaminase, urea dehydroxylase, and azoreduetase the hydrolytic enzymes are P-glucoronidase, P-xylosidase, P-galaetosidase, and a-L-arabinosidase. Studies conducted by Macfarlane and co-workers have shown that proteolysis ean also happen in the colon [31]. More recent findings by this group indieate that bacterial fermentation of proteins in humans could account for 17% of... 

Kerklaan PRM, Bouter S, Koppele JM, et al. 1987. Mutagenicity of halogenated and other substituted dinitrobenzenes in Salmonella typhimurium TA 100 and derivatives deficient in glutathione (TA100/GSH-) and nitroreductase (TA100NR). Mutat Res 176 171-178. 

Mason RP, Holtzman JL. 1975. The mechanism of microsomal and mitochondrial nitroreductase. Electron spin resonance evidence for nitroaromatic free radical intermediates. Biochemistry 14 1626. 

Wardman P, Clarke ED. 1976. Oxygen inhibition of nitroreductase Electron transfer from nitro radical-anions to oxygen. Biochem Biophys Res Common 69 942. 

Wei Cl, Cohen MD, Swartz DD, et al. 1984. Mutagenicity studies of some nitroaromatics with regular Salmonella typhimurium strains and their corresponding nitroreductase-deficient strains. Environ Mutagen 6 410. 

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